Apparent Weightlessness

Problem: How does one simulate weightlessness?

It is commonly believed that because astronauts aboard an orbiting space vehicle appear weightless, the pull of gravity upon them is zero. This condition is commonly referred to as “zero-g”. While it is true that they feel weightless, gravity is acting upon them. It acts with almost the same magnitude as on the earth’s surface.

The key to understanding this condition is realizing that both the astronauts and the space vehicle are in free fall. It is very similar to how you would feel inside an elevator with a snapped cable. The primary difference between the runaway elevator and the space vehicle is that the runaway elevator has no horizontal velocity (relative to the earth’s surface) as it falls toward the earth, so it eventually hits the earth. The horizontal velocity of the space vehicle ensures that as it falls toward the earth it also moves around the earth. It falls without getting closer to the earth’s surface. Both cases involve free fall.

Materials: Styrofoam or paper cup, 2 long rubber bands, 2 washers or other small masses, masking tape, large paper clip

Procedure: 1. Knot together two rubber bands to make one long rubber band. 2. Knot each end around a washer, and tape the washer to the ends. 3. Bore a small hole about the diameter of a pencil through the bottom of a Styrofoam or paper cup. 4. Fit the rubber bands through the hole from the inside. Use a paper clip to hold the rubber bands in place under the bottom of the cup. 5. Hang the washers over the lip of the cup. The rubber bands should be under some tension. See Diagram 6. Drop the cup and record your observations.

Summing Up: 1. Explain why the washers acted as they did when you dropped the cup.

2. Suppose you were standing on a bathroom scale inside an elevator. Based on your observations in this activity, predict what would happen to your weight reading when the elevator: a. Accelerated upward. b. Moved upward at a constant speed.

Adapted with permission from the Comprehensive Conceptual Curriculum for Physics (C3P), Richard P. Olenick, University of Dallas, Irving, TX (2000). Copyright 2000 by Richard P. Olenick, University of Dallas. c. Began to slow down as it prepared to stop on its way to an upper floor. d. Accelerated downward. e. Moved downward at a constant speed. f. Began to slow down as it prepared to stop on its way to a lower floor. g. Free fell after the cable broke.

Adapted with permission from the Comprehensive Conceptual Curriculum for Physics (C3P), Richard P. Olenick, University of Dallas, Irving, TX (2000). Copyright 2000 by Richard P. Olenick, University of Dallas. Apparent Weightlessness Teacher’s Notes

This is an activity for demonstrating the effects of free fall without the mess associated with water (see Now-Newtonian Frame at end of Teacher’s Notes). This activity may be used as a lab and the Non-Newtonian Demonstrator as a teacher demonstration either before or after the lab. It is suggested that every student or pair of students construct the device as directed by the instructor. The minutes spent in this manner are a wise investment in the long run. When everyone has an apparatus that works, the instructor spends less time assisting individuals with construction problem. This device may also be taken home for a family fun physics assignment. If an elevator is available in your school and it is fairly fast, have students take a bathroom scale for a ride. Another option for investigating the elevator questions is to have students visit businesses with elevators and ride the elevators while standing on a scale.

Summing Up: 1. Explain why the washers acted as they did when you dropped the cup. At rest there are two forces acting on the washer - rubber band system, weight due to gravity and the tension in the rubber bands. The tension is caused by the weight of the washers hanging over the cup lip. During the free fall the cup and washers accelerate downward in effect “eliminating” the weight. The tension in the rubber bands, now unbalanced by the weight, pulls the washers into the cup as the bands return to the non-stretched state.

2. Suppose you were standing on a bathroom scale inside an elevator. Based on your observations in this activity, predict what would happen to your weight reading when the elevator: a. Accelerated upward. Ans: Weight would increase. The scale is accelerating into the person’s feet adding to the gravitational force that produces weight. b. Moved upward at a constant speed Ans: The weight would be normal. There is a net force of 0N when an object travels at a constant speed. c. Began to slow down as it prepared to stop on its way to upper floor. Ans: The weight would decrease because the person would still have and upward velocity component and the elevator would be slowing down, decelerating. d. Accelerated downward. Ans: Weight would decrease. The scale is accelerating away from the person’s feet, removing some of the support. e. Moved downward at a constant speed. Ans: The weight would be normal. . There is a net force of 0N when an object travels at a constant speed. f. Began to slow down as it prepared to stop on its way to a lower floor.

Adapted with permission from the Comprehensive Conceptual Curriculum for Physics (C3P), Richard P. Olenick, University of Dallas, Irving, TX (2000). Copyright 2000 by Richard P. Olenick, University of Dallas. Ans: Weight would increase. The scale is decelerating and the person’s feet are pushing harder on the scale adding to the gravitational acceleration that produces weight. g. Free fell after the cable broke. Ans: You are in free fall there is no support force provided by the scale so that it registers a weight. Therefore you experience apparent weightless just as the washers did in the experiment..

Adapted with permission from the Comprehensive Conceptual Curriculum for Physics (C3P), Richard P. Olenick, University of Dallas, Irving, TX (2000). Copyright 2000 by Richard P. Olenick, University of Dallas. Non-Newtonian Frame Demonstration:

Colored water is in the cup.

Use a knife to punch two holes on opposite sides of a Styrofoam cup near the bottom. Fill with water and drop from a height of several meters. During free fall, in accelerated frame of the cup, water exerts no weight on the cup; hence no pressure and stream ceases.

Note: Colored water is much easier for students to see. If you have a video camera, it helps to tape this and show it in slow motion.

Use this demonstration when you are trying to help students understand how a space craft is in free fall, but the objects inside appear to be “weightless”. They are only weightless in relation to each other.

Reprinted with permission: Carpenter and Minnix, DICK and RAE Physics Demo Notebook , (1993). Copyright by Dick and Rae, Inc., Lexington, VA.